The invention relates to a prism system for image inversion in a visual observation beam path. The prism system of the invention can comprise a roof prism with a bottom face, a roof edge inclined with respect to the bottom face, and a gable face that is placed at an angle with respect to the bottom face. The prism system of the invention also can comprise a reflecting prism joined to the gable face of the roof prism. The reflecting prism has a beam-pass face parallel to the bottom face of the roof prism.
A prism system is disclosed by German Patent DE-C 518 143 and is referred to there as a parallel-sight inverting prism system. It has only two beam-pass faces, which form an interface with the air and which are parallel to each other, and six reflective faces, which are arranged in such a way that all the reflections take place in one plane and all are totally reflective. The imaging beams enter the bottom face of the roof prism. The angle between the bottom face of the roof prism and the cemented-on next reflection face is intended to be less than 110.degree.. Such prism systems are normally used in afocal optical imaging systems, such as in telescopes.
In recent years, telescopic instruments, above all field glasses, have increasingly come onto the market. Telescopic instruments have additional functions such as laser distance measurement or inclination and direction measurement (compass). The measurement results preferably are displayed in such a way that they can be read by viewing through the eyepiece. In this case, there are two possibilities, namely, displaying the measurement results at the edge of the image field, outside the actual visual field, or inserting the measurement results into the image field, by superimposing them on the image by means of a beam splitter inserted into the beam path. Inserting the measurement results into the image field is preferred as it is more beneficial in ergonomic terms. Displaying the measurement results at the edge of the image field leads to rotation of the eye when reading. During such reading, since the point of rotation of the eye is typically located more than one centimeter behind the pupil, the pupil of the eye firstly moves out of the exit pupil of the field glass. It is, therefore, necessary for the exit pupil in each case to be sought again by means of parallel displacement of the field glass.
The use of a beam splitter for image superimposition, by contrast, offers the additional possibility of concurrently using the observation beam path for transmission or reception of the laser radiation. A beam splitter can be used in this manner, for example, in an application involving the combination of a field glass with a laser rangefinder. In addition to metallic or dielectric neutral-density filters, dichroic beam splitters are particularly suitable for the beam splitter. Such dichroic beam splitters are particularly suitable because the display wavelength of a display generally emits at the red edge of the visible spectrum, while a possible diode laser preferably emits in the near infrared.
Various solutions have been disclosed as to how insertion of measurement values into an image field by means of a beam splitter can be performed. German Publication DE 41 35 615 A1 discloses the placement of a beam splitter either between the objective and the inversion system or between the inversion system and the eyepiece. If a simple plane plate, with an appropriate reflective layer, is used as a beam splitter, then image errors (astigmatism) must be expected. On the other hand, beam-splitter cubes with a reflective face on a diagonal face are expensive and increase the glass path. In addition, the beam-splitter layers, mostly arranged at 45.degree. to the optical axis, lead to a color distortion profile over the image field.
A further solution is disclosed by German Patent Specification DE 37 04 848 C2. Accordingly to that solution, the insertion is performed directly at an inversion prism built up from three half-cube prisms, in which the beam entry axis is offset vertically with respect to the beam exit axis. For the purpose of insertion into the observation beam path, a beam-splitter coating is applied to a reflective face of the inversion prism, and a half-cube is then cemented on that reflective face. The reflective face, which is at 45.degree. to the beam direction, is used for the image in reflection, while the insertion of the display or of laser radiation is carried out in transmission. Even this solution has disadvantages. Not only does light strike the reflective face at an angle of 45.degree., but, because of the finite opening and of the expanded image field, it strikes at an angle of, for example, 45.+-.5.degree.. In dichroic beam splitters in particular, this leads to a color distortion profile both over the image field and over the pupil. In addition, the reflectance of the beam splitter decreases sharply at the edge of the angular range because of the Brewster effect, particularly for relatively highly reflective glasses, such as BaK4, which are primarily used for inversion prisms in field glasses. This decrease in beam splitter reflectance leads to a drop in brightness.